U.S. Pat. No. 4,479,819 describes the preparation of glass articles exhibiting excellent polarization in the infrared region of the radiation spectrum from glasses containing particles of silver halide dispersed therein, the silver halide being selected from the group consisting of AgCl, AgBr, and AgI. The method disclosed comprised four basic steps:
(1) a batch for a glass containing silver and at least one halide selected from the group consisting of chloride, bromide, and iodide was melted and the melt shaped into a glass body of a desired configuration;
(2) that glass body was subjected to a heat treatment at a temperature at least above the strain point of the glass, but not in excess of 50.degree. C. above the softening point of the glass, for a period of time sufficient to cause the generation of silver halide particles therein selected from the group consisting of AgCl, AgBr, and AgI ranging in size between about 200-5000 .ANG.; thereafter
(3) the glass body was elongated under stress at a temperature above the annealing point of the glass, but below the temperature at which the glass demonstrates a viscosity of about 10.sup.8 poises, such that the silver halide particles were elongated to an aspect ratio of at least 5:1 and aligned in the direction of the stress; and then
(4) the elongated glass body was exposed to a reducing atmosphere at a temperature above about 250.degree. C., but no higher than about 25.degree. C. above the annealing point of the glass, for a period of time sufficient to develop a reduced surface layer on the glass article having a thickness of at least 10 microns (.apprxeq.0.0004") and, preferably, about 50 microns (.apprxeq.0.002"), wherein at least a portion of the elongated silver halide particles was reduced to elemental silver particles having aspect ratios greater than 2:1 which were deposited in and/or upon the elongated particles.
The principal objective of the invention disclosed in that patent was to produce glass articles displaying excellent polarizing properties over the infrared portion of the radiation spectrum, most preferably within the region of 700-3000 nm (7000-30,000 .ANG.), but also up to longer wavelengths, e.g., 3 to 5 microns.
As was explained in the patent, the dichroic ratio is defined as the ratio existing between the absorption of radiation parallel to the direction of elongation and the absorption of radiation perpendicular to the direction of elongation; the sharper (taller and narrower) the peaks, the higher the dichroic ratio. Sharp peaks occur with the presence of relatively small particles. Nevertheless, the patent cautions that the particles must not be too small; with particles smaller than about 100 .ANG., the mean-free-path limitations to the conduction electrons cause the peak to broaden. Moreover, small particles demand very high elongation stresses to develop the necessary aspect ratio. And, because the likelihood of glass body breakage during a stretching-type elongation process is directly proportional to the surface area of the body under stress, there is a very practical limitation as to the level of stress than can be applied to a glass sheet or other body of significant bulk. In general, a stress level of a few thousand psi has been deemed to comprise a practical limit.
It was emphasized that the heat treating parameters of Steps (2), (3), and (4) were critical to securing the desired properties in the final product. To illustrate:
The growth of silver halide particles cannot occur at temperatures below the strain point of the glass because the viscosity of the glass is too high. Therefore, crystallization temperatures above the annealing point are preferred and, where physical support is provided for the glass body, temperatures up to 50.degree. C. above the softening point of the glass can be employed.
Elongation of the glass body, along with the silver halide particles grown in Step (2), will be carried out at temperatures above the annealing point of the glass, but below the softening point thereof. Thus, a temperature at which the glass exhibits a viscosity of about 10.sup.8 poises had been adjudged to constitute the maximum. Customarily, the elongation process will be conducted at temperatures at least 50.degree. C. below the softening point of the glass to permit high stresses to be developed and to inhibit respheroidization of the silver halide particles.
Laboratory investigations indicated that silver halide particles can be elongated at lower stresses than metallic silver particles, but yet will provide excellent polarization characteristics after reduction to elemental or metallic silver. Nonetheless, firing of the elongated body in a reducing environment under atmospheric conditions will be undertaken at temperatures above 250.degree. C., but no higher than 25.degree. C. above the annealing point of the glass, and, preferably, somewhat below the annealing point of the glass, to prevent any proclivity of the particles to respheriodize.
Finally, experience had demonstrated that the silver halide crystals generated during the initial heat treatment [Step (2)]ought to have diameters of at least about 200 .ANG. in order to assume, upon elongation, aspect ratios of at least 5:1 such that, upon reduction to elemental silver particles, those latter particles will display aspect ratios greater than 2:1, thereby assuring the placement of the long wavelength peak at least near the edge of the infrared region of the radiation spectrum, while avoiding serious breakage problems during the subsequent elongation step. At the other extreme the diameters of the initial silver halide particles ought not to exceed about 5000 .ANG. in order to preclude the development of significant haze in the glass accompanied with a decreased dichroic ratio resulting from radiation scattering effects.
Laboratory investigations and field experience have evidenced that one of the key measures of the effectiveness of the above-described polarizing bodies is the contrast ratio, or as referred to simply in the art as contrast. Contrast comprises the ratio of the amount of radiation transmitted with its plane of polarization perpendicular to the elongation axis to the amount of radiation transmitted with its plane of polarization parallel to the elongation axis. In general, the greater the contrast, the more useful (and, hence, more valuable) the polarizing body.
Another important feature of a polarizing body is the bandwidth over which it is effective. The polarizing glass articles produced in accordance with the description of the above patent tend to have a rather narrow band over which the contrast is at a maximum. Thus, on either side of that peak wavelength the contrast falls off quite sharply.
Laboratory experimentation has indicated that the level of contrast attainable in the polarizing glass bodies prepared in accordance with the above patent is dependent upon, among other things, the amount of reduction occurring during Step (4), i.e., during the reduction firing step. Typically, the greater the extent of reduction, the greater the level of contrast. It has been demonstrated that contrast can be increased by employing higher reducing firing temperatures and/or longer periods of firing. That practice is limited, however, inasmuch as higher temperatures and/or longer exposure times lead to respheroidization of the silver halide particles wherein the elongated particles shrink and/or break apart, thereby tending to form spheres. Such respheroidization can result in a decrease in contrast and/or a narrowing of the peak absorption band or a shifting of the peak absorption band in the direction of shorter wavelengths. To illustrate, the "standard" process for preparing polarizing glass articles according to the above patent has utilized firing in a hydrogen atmosphere for four hours at 425.degree. C. When the glass articles were fired for seven hours in a hydrogen atmosphere at 425.degree. C., the contrast exhibited by the articles was increased somewhat, but with a concurrent reduction in the bandwidth of high contrast.
Consequently, the primary objective of the present objective of the present invention was to prepare infrared polarizing glass bodies of higher contrast and greater bandwidth than those produced in U.S. Pat. No. 4,479,819.
A second objective of the present invention was to produce infrared polarizing glass bodies exhibiting high contrast over a relatively broad bandwidth utilizing shorter exposure periods to reduction firing than required in the method of U.S. Pat. No. 4,479,819.